1. Field of the Invention
The present invention relates to a television tuner capable of receiving a television signal as well as an FM broadcast signal.
2. Description of the Prior Art
A television signal and an FM broadcast signal have different bandwidths. The television signal has a bandwidth of 6 MHz, and the FM broadcast signal has a bandwidth of 200 kHz. In addition, the television signal and the FM broadcast signal have different reception levels. The television signal has a reception level about 6 dB higher than that of the FM broadcast signal.
Conventionally, when a television signal and an FM broadcast signal are received by one television tuner, an SAW filter for a television signal and an SAW filter for an FM broadcast signal are provided in the intermediate-frequency circuit of a television tuner. When a television signal is received, the SAW filter for receiving a television signal eliminates an interference wave. When an FM broadcast signal is received, the SAW filter for receiving an FM broadcast signal eliminates an interference wave.
The construction of a conventional television tuner will be described in accordance with FIG. 8. In FIG. 8, a television signal or an FM broadcast signal received by an antenna 71 is tuned by an antenna tuned circuit 72, and then, is amplified by a high-frequency amplifier 73 to be tuned by a high-frequency tuned circuit 74. The antenna tuned circuit 72 and the high-frequency tuned circuit 74 have a varactor diode, not shown, respectively. An oscillation frequency control voltage is applied from a PLL circuit 82 described later to the cathode of the respective varactor diodes, so that the tuning frequency is controlled to be a frequency to be received.
A mixer 75 is fed the television signal or the FM broadcast signal outputted from the high-frequency tuned circuit 74, and is fed a local oscillation signal from a local oscillator 81. The local oscillator 81 is controlled by the oscillation frequency control voltage inputted from the PLL circuit 82 so as to output a local oscillation signal of the center frequency of the television signal to be received or of a frequency about 44 MHz which is higher than the FM broadcast signal. Thus, the mixer 75 frequency-converts the television signal to the intermediate-frequency band having a center frequency of 44 MHz, and frequency-converts the FM broadcast signal to 44 MHz. The oscillation frequency control voltage outputted from the PLL circuit 82 is applied to the antenna tuned circuit 72 and the high-frequency tuned circuit 74, as described above.
The television signal or the FM broadcast signal frequency-converted by the mixer 75 is tuned by a intermediate-frequency tuned circuit 76 to be inputted to a change-over switch 77. The change-over switch is controlled by a band decoder 83. When the television signal is received, the change-over switch 77 connects the intermediate-frequency tuned circuit 76 and an SAW filter for a television signal 78, and the television signal passes through the SAW filter for a television signal 78. The bandwidth of the SAW filter for a television signal 78 is 6 MHz. On the other hand, when the FM broadcast signal is received, the change-over switch 77 connects the intermediate-frequency tuned circuit 76 with an intermediate-frequency amplifier 79, and the FM broadcast signal is amplified by the intermediate-frequency amplifier 79 to eliminate a difference in levels of the television signal and the FM broadcast signal. Then, the FM broadcast signal passes through an SAW filter for an FM broadcast signal 80. The bandwidth of the SAW filter for an FM broadcast signal is 1.5 MHz.
Such a conventional television tuner must be provided with SAW filters having different bands separately for a television signal and an FM broadcast signal in order to receive the television signal and the FM broadcast signal. This prevents the television tuner from being made smaller and inexpensive.
Accordingly, an object of the present invention is to provide a television tuner which reduces the number of parts such as SAW filter, can be made smaller, and is inexpensive.
To solve the foregoing problems, a television tuner of the present invention comprises a mixer to be fed a television signal or an FM broadcast signal for frequency-converting the television signal to the intermediate-frequency band of a television, and frequency-converting the FM broadcast signal to a voice intermediate frequency in the intermediate-frequency band, and an intermediate-frequency tuned circuit provided at a later stage than the mixer, wherein the intermediate-frequency tuned circuit comprises a first parallel tuned circuit tuned to a frequency substantially in the center of the intermediate-frequency band, a second parallel tuned circuit tuned to a frequency lower than the voice intermediate frequency, and switch means connected in series with the second parallel tuned circuit, a signal of the intermediate-frequency band and a signal of the voice intermediate-frequency band are applied to both ends of the first parallel tuned circuit and both ends of a series circuit comprising the second parallel tuned circuit and the switch means, when the television signal is received, the switch means is brought into an off state, when the FM broadcast signal is received, the switch means is brought into an on state, so that a parallel circuit comprising the first tuned circuit and the second tuned circuit is tuned to the voice intermediate frequency. The switch means is switched on or off to change the tuning frequency and the bandwidth of the intermediate-frequency tuned circuit.
A television tuner of the present invention comprises a mixer to be fed a television signal or an FM broadcast signal for frequency-converting the television signal to the intermediate-frequency band of a television, and frequency-converting the FM broadcast signal to a voice intermediate frequency in the intermediate-frequency band, and an intermediate-frequency tuned circuit provided at a later stage than the mixer, wherein the intermediate-frequency tuned circuit comprises a first parallel tuned circuit, a second parallel tuned circuit, and a first varactor diode connected in series with the second parallel tuned circuit, the parallel tuning frequency of any one of the first parallel tuned circuit and the second parallel tuned circuit is set to be higher than the frequency in the center of the intermediate-frequency band, and the parallel tuning frequency of the other tuned circuit is set to be lower than the voice intermediate frequency, the signal of the intermediate-frequency band and the signal of the voice intermediate-frequency band are applied to both ends of the first parallel tuned circuit and both ends of a series circuit comprising the second parallel tuned circuit and the first varactor diode, when the television signal is received, a reverse bias voltage is applied to the first varactor diode, so that the second parallel tuned circuit and the first varactor diode are series-tuned to the frequency of the intermediate-frequency band corresponding to the image carrier of a higher side television channel adjacent to a television channel to be received, and a parallel circuit of the second parallel tuned circuit and the first varactor diode connected in series, and the first parallel tuned circuit is tuned to the intermediate-frequency band, when the FM broadcast signal is received, the first varactor diode is brought into a conductive state, so that a parallel circuit comprising the first parallel tuned circuit and the second parallel tuned circuit is tuned to the voice intermediate-frequency. The bias voltage of the first varactor diode is switched to change the tuning frequency and the bandwidth of the intermediate-frequency tuned circuit.
The television tuner of the present invention comprises an intermediate-frequency amplifier provided at a later stage than the mixer, and a series tuned circuit connected between the output end and the ground of the intermediate-frequency amplifier, wherein the series tuned circuit has a midpoint as an output end, the tuning frequency of the series tuned circuit can be changed, when the television signal is received, the tuning frequency of the series tuned circuit is outside the intermediate-frequency band, when the FM broadcast signal is received, the tuning frequency of the series tuned circuit is tuned to the voice intermediate frequency. The levels of the television signal and the FM broadcast signal are adjusted.
In the television tuner of the present invention, the first parallel tuned circuit has a first inductance element and a first capacitor connected in parallel with each other, the second parallel tuned circuit has a second inductance element and a second capacitor connected in parallel with each other, the series tuned circuit has a second varactor diode, a third inductance element, and a third capacitor connected in series with each other, and a fourth capacitor connected in parallel with the third capacitor through second switch means, the series tuned circuit has a midpoint as an output end, when the television signal is received, a reverse bias voltage is applied to the first varactor diode and the second varactor diode, and the second switch means is brought into an off state, when the FM broadcast signal is received, a forward bias is applied to the first varactor diode and the second varactor diode, and the second switch means is brought into an on state.
In the television tuner of the present invention, the intermediate-frequency tuned circuit is provided with a trap circuit, the trap circuit has a fourth inductance element and a third varactor diode connected in parallel with each other, when the television signal is received, a reverse bias voltage is applied to the third varactor diode, so that the trap circuit attenuates the signal of the frequency of the intermediate-frequency band corresponding to the voice carrier of a lower side television channel adjacent to a television channel to be received, when the FM broadcast signal is received, the reverse bias voltage applied to the third varactor diode is lowered than when the television signal is received, and the trap frequency is a frequency between the image carrier and the voice carrier of the intermediate-frequency band. The television signal and the FM broadcast signal have a suitable band characteristic, respectively.